Solutions of poly(p-phenylene benzobisthiazole) (PBZT) in methane sulfonic acid (MSA) were prepared and studied. Solutions with concentrations less than 0.04 wt % PBZT were characterized by dilute solution viscometry. Planar PBZT waveguides were spin-coated from a 0.5 wt % PBZT solution onto oxidized silicon wafers. The optical attentuation of the resulting polymer waveguides was measured and found to depend on both the thickness of the oxide layer on the silicon substrate and also the wavelength of the incident light. The lowest optical loss recorded for PBZT in this investigation was 4.81 +/- 1.39 dB/cm at 834 nm. This work thus demonstrates the successful fabrication of PBZT into thin-film planar waveguides. The PBZT films prepared here also show improved optical characteristics over PBZT films prepared previously by either extrusion or spin coating. (C) 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1448-1456, 2000.